Instead of specifying BOOST_VERSION which applies to both the download URL and the source code itself, now specify BOOST_SRC_VERSION. BOOST_SRC_VERSION specifies where to download the source (e.g. the source tarball download link), and once it is downloaded and extracted, the boost.sh script extracts the actual library version from <src-dir>/boost/version.hpp. The only effect this change has is to request the source version via a different environment variable name. The purpose of this feature is to support the subsequent addition of a feature to optionally retrieve the Boost source code from the Boost Subversion repo by specifying either a repo revision number or the "HEAD" special value. These specifiers have no information about the library version, so in these cases the actual library version *must* be extracted from the Boost library source code.
For the "extract version from source" feature, revert the public-facing variable name from the changed BOOST_SRC_VERSION back to the original BOOST_VERSION. The attempt to clarify its modified purpose was not very successful, certainly not to the extent that it justified changing the public interface to the script. The internal variable used by the script is now BOOST_HPP_VERSION.
Merge pull request #6 from morsen/extract-version-from-source

Extract Boost version from source header